Electronic ballast circuits for powering gas discharge lamps have been studied for some time. These circuits, however, have certain drawbacks, as discussed, for example, in U.S. Pat. No. 4,972,124 to Charles D. Powers. In this patent, Powers discloses a circuit which was designed to overcome these drawbacks. However, his circuit is designed for series loads and is not suitable for multi-parallel loads. In Powers' design, a capacitor is used for the ballasting and current limiting element. The use of a capacitor as the ballasting element results in a large imaginary current being fed back to the output transformer requiring the voltampere (VA) rating of this transformer to be large. Consequently, the physical size of the output transformer must also be large, thereby reducing the overall efficiency of the ballast and increasing its cost. The use of a capacitor as the ballasting element also limits the operating frequency of the ballast to approximately 30 Khz, whereas a low crest factor and high efficiency are required.
Gas discharge lamps normally present the powering electronic ballast with a very harsh environment that includes high temperature, and high in-rush voltage and current. Operation for long periods of time in such a harsh environment requires that the electronic ballast be of very robust design. Therefore, in the ballast, the transistor has to be well protected from all kinds of high voltage and high current. In Powers' design there is not enough protection circuitry to reduce the possibility of damage to the transistors. Thus, the reliability of the design is inadequate.